There are literally hundred of UML CASE tools and thus it is very difficult to find any relevant comparison among them (and I am definetely not planning doing that either). I've been using myself MagicDraw (this is the one I currently use), Poseidon and Objecteering with similar results. I've also used ArgoUML and a couple of Eclipse UML2 compatible tools but, in my opinion, they still need to work a little bit more in improving their usability.

Btw, when selecting a tool my basic recommendation is to think ahead how do you plan to use it. If you plan to do some code-generation make sure the tool supports the platforms you work with. If you want to draw some informal models to facilitate the discussion among the developers focus on its usability (and maybe you don't even need a UML CASE tool and a drawing tool would be more than enough). 

For more info on UML tools you should also check the series of deep interviews with UML/MDE tool creators to know more about how and why these kinds of tools are being developed here and the blog posts announcing and analyzing modeling tools

In what follows, I'll try to provide several lists of UML CASE tools, classified in the following categories: general CASE tools, textual UML tools , Eclipse UML2 compatible tools and open source tools.

Generic CASE tools:

• Vendor directory listing by the OMG: Listing of vendors, specially of those that somehow collaborate with the OMG.
• Objects by design list of tools: Great list but unfortunately also outdated
• Comparison of UML tools in Wikipedia
• Mario Jeckle's list of tools: List of 100 UML CASE tools. The comparison is unfortunately outdated.
• UML Forum - tools: simple comparison of some well-known tools

Textual UML tools:

There are also several tools that support the use of textual notations/languages to describe UML models (and automatically display the corresponding graphical UML diagram). List of tools:

• yUML is an online service for creating class and use case diagrams. The service can be called from your blog or web page (with the textual description as part of the URL)to automatically display the image when accessing it. We can now generate yUML models from other UML modeling tools.
• UML Graph automatically draws class and sequence diagrams. For the class diagrams, uses a Java-based syntax complemented by javadoc tags. Running the UmlGraph doclet on the specification will generate a Graphviz diagram specification. For sequence diagrams uses a different approach (and this is one aspect I don't like about the tool, you are basically working with two different tools here). Pic macros are used to define objects and method invocations. Then, the pic2plot program processes the macros to generate PNGs and other graphic formats. LightUML integrates UMLGraph in Eclipse.
• TextUML Toolkit is another recent development. It is an open-source IDE that uses the UML viewer that is part of EclipseGraphviz project to display the models (basically class diagrams, ongoing work on activity diagrams).
• MetaUML is a "library for typesetting UML diagrams, using a human-friendly textual notation". It currently supports class, activity and use case diagrams and state machines. Their main goal is to create UML diagrams readily usable in a LaTeX document.
• modsl is UML modeling library for text-to-diagram translation. So far, it supports class diagrams and collaboration diagrams. In theory, as with the yuml tool, you should be able to write the model description as part of the text of your wiki/blog/... and then use their online service to automatically render the corresponding image (but this was not working at the time of writing this post).
• USE also uses a textual UML model description (including OCL constraints) as input but in this case the goal is not to visually display the model but to animate the model by creating and manipulating snapshots of the model that allow designers to validate the model and its constraints.
• PlantUML supports use case, class, activity and sequence diagrams. Integration with Eclipse and Microsoft Word. See a more detailed description
• WebSequenceDiagrams and Quick Sequence Diagrams Editor focus only on the generation of sequence diagrams.
• Simple DSL for sequence diagrams based on UMLGraph . Transformation of the diagram to UMLGraph notation is done using the TXL Programming Language
• (only for hardcore latex fans) Latex macros for drawing UML Sequence diagrams , the name says it all
• Diagrammr focuses on the generation of diagrams from simple sentences (of type "A verb B"). Thus, it is not specifically a tool for the textual specification of UML diagrams though the graphical representation resembles UML sequence diagrams. See an example
• metaDepth is a textual framework for multi-level meta modeling

Eclipse UML tools:

It is also worth to take a look at the list of Eclipse UML2 compatible tools . As we commented here , the wide adoption of eclipse-based tools could turn the XMI format they use in a kind of de facto standard that all other tools should be compatible with.

Open source UML tools:

Finally, I'd like to mention a couple of lists of open source UML tools (note that the lists are not complete, for instance, moskitt , UMLet and diagram designer are missing).

• Free open source UML tools from DevCurry
• Survey of Open Source Modeling Tools for Software Development
• Top 5 open source UML tools from Open Tube.
• Best Open Source UML tools from Open Source Applications

Tools specific for MACs:

• MacA&D with support for all major UML 2.2. diagrams and code-generation capabilities for SQL, C++, Objective-C, Java, PHP and REALbasic
• CanvasUML : free but only for class diagrams
• Other modeling tools like QuickUML, MagicDraw and VisualParadigm have a version for MAC.

Tools for mobile devices:

• Mock4U : Using Balsamiq to mock UML models in your iPhone thanks to this free Mock4U UML extension and the iPhone templates available in Balsamiq
• iUML : NOT really a UML tool but a UML documentation guide for your iPhone that can be handy as a reference when drawing UML diagrams.
• UML Process Aid is another UML documentation tool for your iPhone.

Do you know other tools/lists? Let us know!